The ribulose monophosphate pathway is known for C1 metabolism of methylotroph bacteria. This pathway comprises three stages starting from fixation of formaldehyde with ribulose 5-phosphate, followed by cleavage of fructose 6-phosphate and regeneration of ribulose 5-phosphate. The ribulose monophosphate pathway is a pathway which is coupled to several metabolic systems. The gene structures of respective enzymes involved in this pathway have been of great interest. However, few reports have been submitted on this pathway, and little genetic analysis has been achieved.
3-Hexulose-6-phosphate synthase (hereinafter referred to as "HPS", if necessary), which catalyzes the initial reaction of the ribulose monophosphate pathway, has been already purified from Methylomonas aminofaciens which is a Gram-negative obligate methanol-assimilating bacterium. A gene coding for this enzyme has been cloned, and its primary structure has been reported (Yanase, H. et al., FEMS Microbiol. Lett., 135, 201-205 (1996)).
By the way, the biochemical substance, in which a specific position on the objective compound molecule is labeled with a stable isotope element, i.e., carbon 13 (.sup.13 C), is useful for the study of metabolic pathway of the organism. Recently, the investigation of the situation of metabolic substances in vivo based on the use of the carbon 13-NMR is an extremely important technique for diagnosis of various diseases and daily health examination. It is necessary, or it has been demanded, for such a new technique, to inexpensively supply a substance in which a certain objective position is labeled with carbon 13.
The present inventor has been established a method for preparing [1-.sup.13 C]D-glucose 6-phosphate from carbon 13-labeled methanol by utilizing the formaldehyde-fixing pathway of a methanol-assimilating bacterium. However, the yield of synthesis of the objective compound has not been so high (Biosci. Biotech. Biochem., 5, 308-312 (1993)).
Therefore, it has been demanded to obtain a system which makes it possible to efficiently produce only the objective product. A method is conceived as one of such systems, in which a series of respective enzymes for synthesizing labeled D-fructose 6-phosphate by using labeled formaldehyde and ribulose 5-phosphate are prepared, and a reaction system is constructed based on the use of the enzymes to efficiently prepare the objective labeled compound. As for HPS which is the initial enzyme in the reaction system, its gene has been isolated from Methylomonas aminofaciens as described above, and its structure has been also clarified. However, hexulose-phosphate isomerase (hereinafter referred to as "HPI" or phospho-3-hexuloisomerase (PHI), if necessary), which is an enzyme to catalyze the next step reaction, has been purified only partially, and neither its amino acid sequence nor gene structure has been known. Further, neither amino acid sequence nor gene structure has been reported for both enzymes of HPS and HPI of Gram-positive bacteria including the genus Mycobacterium as facultative methanol-assimilating bacteria.
In the circumstance as described above, in order to establish a system for efficiently preparing an objective labeled compound, it has been demanded to isolate a gene coding for HPI and provide a method for efficiently producing HPI based on the use of the gene.